1. Field of the Invention
The present invention relates to an image pickup lens and an image pickup apparatus, and more particularly to a wide angle image pickup lens which is appropriate for use with a vehicle camera, a surveillance camera, or the like which includes an image sensor, such as a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor), and an image pickup apparatus having the image pickup lens.
2. Description of the Related Art
Image sensors, such as CCD or CMOS devices, have recently been greatly downsized with increased pixel counts. Consequently, image pickup devices themselves and image pickup lenses to be mounted on the devices are also required to be reduced in size and weight. In the mean time, image pickup lenses used in vehicle cameras and surveillance cameras are required to have a high weather resistance in addition to a wide angle of view and high optical performance to secure a favorable field of view in a wide range.
Further, due to demand for cost reduction for the manufacture of image pickup lenses used in the aforementioned applications, an optical system with less number of lenses is sought. Conventional image pickup lens with four lenses are described, for example, in Japanese Unexamined Patent Publication No. 2002-244031 (Patent Document 1), U.S. Pat. No. 7,280,289 (Patent Document 2), U.S. Pat. No. 7,375,906 (Patent Document 3), Japanese Unexamined Patent Publication No. 2005-227426 (Patent Document 4), and U.S. Pat. No. 7,518,809 (Patent Document 5).
Recently, there has been a growing demand for wider angles, such as a demand for a total angle of view of over 180° in the application fields of vehicle cameras, surveillance cameras, and the like. Recently, there has also been a growing demand for image pickup lenses having high resolution and high optical performance to obtain a favorable image in a wide range of imaging area along with the downsizing of image sensors with increased pixel counts. It has been difficult, however, for conventional lens systems to realize both a compact and inexpensive structure and to satisfy the recent demands of wider angle and high optical performance.
Patent Document 1 describes, as a wide angle lens of Example 3, a lens system that includes four lenses disposed from the object side in the order of first to fourth lenses with an aperture between the third and fourth lenses. Although, Patent Document 1 does not describe the F-number and the angle of view of the lens system, it is quite unlikely that the lens system complies with a specification of over 180° of total angle of view when considering that the refractive index of the first lens is about 1.52 and the first and second lenses have relatively small negative powers.
Lenses described in Patent Documents 2 and 3 have total angles of view of about 140° to 165° and about 152° to 164° respectively. Thus, the lenses can not comply with the recent demand of wider angles of view over 180°. The lens described in Patent Document 4 has an F-number of 2.5 to 2.8 with a total angle of view over 180°, but when a projection system with an ideal image height of 2×f×tan (φ/2), wherein f is the focal length of the entire system and φ is the half angle of view, is employed, the system has a disadvantage that the distortion increases rapidly and significantly on the negative side when the half angle of view exceeds 80° and an image in the outermost peripheral portion becomes small. Patent Document 5 describes an example having a total angle of view close to 190° with distortion and chromatic aberration of magnification being corrected satisfactorily, but astigmatism remains. Therefore, when the lens is used in conjunction with an image sensor having a high pixel count, the lens may be required a wider field of depth.
In view of the circumstances described above, it is an object of the present invention to provide a compact and inexpensive image pickup lens, yet capable of realizing high optical performance, and an image pickup apparatus having the image pickup lens.